Naturally occurring sulfur compounds, especially organic sulfur compounds, are commonly present in various oil products, such as vegetable, mineral and animal oils. These sulfur compounds include, for example, mercaptans, sulfides, polysulfides, sulfoxides, thiophenes and thiophanes. In order to stabilize and upgrade the properties of these oil products, most of them undergo a hydrogenation process to convert unsaturated hydrocarbons, alcohols, acids, fats to their respective saturated compounds. The hydrogenation process usually occurs over a Group VIII metal catalyst, particularly nickel, cobalt, platinum or palladium catalysts. These Group VIII catalysts are particularly sensitive to sulfur poisoning. Even when only trace levels of sulfur compounds in the range of 1 to 2 parts per million ("ppm") and lower are present in the oils, the hydrogenation catalysts can significantly decrease or completely lose their activity in a short period of time. Consequently, substantial efforts have been undertaken to eliminate or bind these sulfur contaminants to adsorbents prior to the oil hydrogenation process.
Various adsorption processes are well known for the removal of these sulfur compounds from catalytic process feed streams. In one conventional process, adsorption products are placed upstream to remove the sulfur compounds prior to interaction of the oils with the catalysts. Chemisorption of sulfur-containing compounds using metal or metal oxide adsorbents is the most popular method used for removal of these sulfur compounds from these oils. These metal oxides include nickel, platinum, cobalt, or copper in zerovalent form or zinc, manganese, cadmium, or copper oxides, either alone or secured to a support system.
For example, U.S. Pat. No. 4,634,515 discloses a sulfur trap adsorbent for sulfur-sensitive reforming catalyst protection, which comprises nickel on a support. At least 50% of the nickel is in a reduced, zerovalent state. The catalyst has a surface area of 30-50 m.sup.2 /g. U.S. Pat. No. 4,204,947 describes copper metal, copper oxide, or copper chromite secured on an inorganic, porous carrier as an adsorbent for the removal of mercaptans from hydrocarbon oils. This sulfur scavenger has a surface area in the range of 20-1000 m.sup.2 /g and provides sulfur content reduction in a product from 2-3 ppm to 0.3 ppm range. U.S. Pat. No. 4,179,361 describes an adsorbent for mineral oil purification, which comprises cobalt oxide on a porous alumina. U.S. Pat. No. 4,225,417 discloses the use of manganese or manganese oxide on a support (clay, graphite, alumina, etc.) for sulfur scavenging and catalytic reforming catalyst protection.
All adsorbents mentioned above provide a high level of sulfur removal. These metal and metal oxide components bind organic sulfur compounds which results in metal sulfide formation. One disadvantage is that they must be used with expensive components in significant quantities. Further, they are nonregenerable or are very difficult to regenerate. This feature increases the operational expense for the catalytic process feed purification.
In an effort to decrease the adsorbent costs, the use of multistage purification with different adsorbent beds has been suggested. For example, U.S. Pat. No. 4,446,005 proposes a guard bed for reforming catalysts, which comprises two components: nickel metal on an activated alumina or alumosilicate and copper, zinc, or chromium oxides on a porous support. U.S. Pat. No. 5,322,615 discloses nickel or platinum on alumina as a first step adsorbent and potassium on alumina as a second step adsorbent. U.S. Pat. No. 5,106,484 teaches a three stage purification. A NaY synthetic zeolite is used at a temperature below 45.degree. C. in the first step. Massive nickel or nickel on activated alumina maintained at a temperature of about 150.degree. C. is used as the second step and the final third step employs a manganous oxide adsorbent operated at temperatures of 425-600.degree. C. Although such multistage beds decreases the cost of the metal utilized, due to the entirely different temperature conditions at each stage, the process is quite complicated in the purification process technique and technology.
Several types of adsorbents for the intensive recovery of sulfur compounds have been formed which are based on synthetic zeolites, particularly in a transition metal-exchanged form. U.S. Pat. No. 4,188,285 discloses an adsorbent for thiophens for gasoline purification, which comprises a silver-exchanged form of an ultrastable faujasite Y. U.S. Pat. No. 5,057,473 discloses a desulfurization adsorbent, which comprises mono-(copper) or bication (copper-lanthanum) exchanged forms of a molecular sieve X. U.S. Pat. No. 5,146,036 describes the use a zeolite containing copper, silver, zinc, or mixtures thereof for the low-level recovery of sulfides or polysulfides. U.S. Pat. No. 5,807,475 teaches the use of nickel- or molybdenum-exchanged forms of zeolites X and Y for the removal of thiophens and mercaptans from gasoline. U.S. Pat. No. 5,843,300 discloses a regenerable adsorbent for organic sulfur compounds removal from petroleum feedstocks, which comprises a potassium form of a zeolite X loaded with 0.05-1% wt of palladium or platinum metal.
Although zeolite adsorbents provide a high level of sulfur recovery, their adsorption capacity is very low. For example, the adsorption capacity of the adsorbents, according to U.S. Pat. Nos. 5,146,039 and 5,807,475, does not exceed 0.1% w., even at relatively high concentrations of sulfides and thiophens. Further, the adsorption capacity of the adsorbent of U.S. Pat. No. 5,843,300, is only 0.6% w. for isobuthyl mercaptan and 0.3% for 2-methyl thiophene at an initial concentration of sulfur impurities of 3700 ppm (see Examples 1 and 2 of U.S. Pat. No. 5,843,300).
Application, Ser. No. 09/316,842, filed May 21, 1999, titled, "Molecular Sieve Adsorbent-Catalyst for Sulfur Compound Contaminated Gas and Liquid Streams and Process for Its Use" owned by the assignee discloses a high capacity adsorbent, which comprises synthetic faujasites, X, Y, or LSF in exchanged forms of bivalent cations of transition metals, preferably copper, zinc, cadmium, or manganese.
The main disadvantage of prior art adsorbents for oil hydrogenation catalyst protection, is their slow rate of adsorption of sulfur-contaminated compounds. The rate of organic sulfur compound chemisorption on transition metal oxides or molecular sieves is usually 20-50 times slower than the adsorption rate of the Group VIII metal catalysts for the sulfur compounds. Because of this deficiency, the adsorbents of the prior art do not protect the hydrogenation catalysts from sulfur poisoning "in situ" or in one bed. Thus, they are conventionally used in a preliminary feed treating stage, most commonly in a special adsorber. This complicates the hydrogenation process and substantially increases capital and operational costs.
While these products have been useful for hydrogenation catalyst protection against sulfur poisoning, it is important to create new adsorbents which overcome the disadvantages of the prior art adsorbents.
It is therefore an aspect of the invention to produce a novel adsorbent for organic sulfur compounds with improved efficiency in the protection of oil hydrogenation catalysts.
It is a further aspect of the invention to produce a zinc-exchanged form of a low silica faujasite (LSF) adsorbent in an which zinc cations are present in inequivalent excess.
It is a still further aspect of the invention to provide an adsorbent for sulfur compounds with an adsorption rate equal to or higher than the adsorption rate for sulfur compounds of conventional hydrogenation catalysts.
It is a still further aspect of the invention to provide an adsorbent for sulfur compounds with the capability to absorb very low quantities of sulfur compounds in oils down to a level of 1 ppm or lower.
It is a still further aspect of the invention to provide an adsorbent, which provides enhanced adsorption capacity for organic sulfur compounds at the elevated temperatures normally used for oil hydrogenation process.
It is a still further aspect of the invention to provide a relatively low cost adsorbent for use with a hydrogenation catalyst which does not substantially increase the cost of the overall oil hydrogenation process.
Still further objects and advantages will become apparent from consideration of the ensuing description of a preferred embodiment of the invention and examples therewith.